Ultra Wideband (UWB) technology, which uses base-band pulses of very short duration to spread the energy of transmitted signals very thinly from near zero to several GHz, is presently in use in military applications. Commercial applications will soon become possible due to a recent Federal Communications Commission (FCC) decision that permits the marketing and operation of consumer products incorporating UWB technology.
The key motivation for the FCC's decision to allow commercial applications is that no new communication spectrum is required for UWB transmissions because, when they are properly configured, UWB signals can coexist with other application signals in the same spectrum with negligible mutual interference. In order to ensure negligible mutual interference, however, the FCC has specified emission limits for the UWB applications. For example, a basic FCC requirement is that UWB systems do not generate signals that interfere with other narrowband communication systems.
The emission profile of a UWB signal can be determined by examining its power spectral density (PSD). The PSD for ideal synchronous data pulse streams based upon stochastic theory is well known and is described in an article by M. Z. Win, entitled “Spectral Density of Random Time-Hopping Spread-Spectrum UWB Signals with Uniform Timing Jitter”, Proc. MICOM'99, vol. 2, pp. 1196–1200, 1999. This article also provides a characterization of the PSD of the Time-Hopping Spread Spectrum signaling scheme in the presence of random timing jitter using a stochastic approach.
The power spectra of UWB signals consist of continuous and discrete components. Generally speaking, discrete components contribute more to the PSD than continuous components, which behave as white noise. Thus, discrete components cause more interference to narrowband wireless systems than continuous components. Accordingly, a basic objective in the design of UWB systems is to reduce the discrete component of the UWB power spectra. Another objective for UWB systems is to increase the power efficiency.
UWB communication system currently use one of two modulation techniques. These techniques include a pulse position modulation (PPM) technique and a bi-phase shift keying (BPSK) technique. The PPM technique has good power efficiency but a relatively high PSD. The BPSK technique, on the other hand, has a relatively low PSD but low power efficiency.
There is an ever present desire for efficient communication systems that transmit signals with low PSD. Accordingly, there is a need for improved modulation methods, apparatus, and systems that are not subject to the above limitations. The present invention fulfils this need among others.